1. Field of the Invention
The present invention relates to a zinc phosphate chemical conversion film, more particularly to the composition thereof, and to a method for forming the same.
2. Description of the Related Art
The conventionally known zinc phosphate chemical conversion film is formed by a high temperature bath having a temperature of 40.degree. C. or higher, and the chemical composition thereof consists of Zn.sub.3 (PO.sub.4).sub.2.4H.sub.2 O (hopeite tetrahydrate), and Zn.sub.2 Fe(PO.sub.4).sub.2.4H.sub.2 O (phosphophillite tetrahydrate).
When Zn.sub.3 (PO.sub.4).sub.2.4H.sub.0 in the above conventional zinc phosphate chemical conversion film is heated to a temperature of from 100.degree. to 120.degree. C., the water of crystallization is eliminated to yield Zn.sub.3 (PO.sub.4).sub.2.2H.sub.2 O.
Journal of the Iron and Steel Institute of Japan (Tetsu to Hagane) 69 (13), 121, 1983 states, on page 121, that the Zn.sub.3 (PO.sub.4).sub.2.4H.sub.2 O in the above mentioned film is converted to Zn.sub.3 (PO.sub.4).sub.2.2H.sub.2 O due to a high temperature of 100.degree. C. or higher generated at the time of baking the paint. When the change of Zn.sub.3 (PO.sub.4).sub.2.4H.sub.2 O to Zn.sub.3 (PO.sub.4).sub.2.2H2O takes place, as referred to in the above journal, the vacancies increase in the chemical conversion film correspondingly to the desiccation of two moles of water of crystallization from Zn.sub.3 (PO.sub.4).sub.2.4H.sub.2 O, or alternatively, the structure of zinc phosphate chemical conversion film changes.
Accordingly, problems arise such that the adhesivity of the zinc phosphate chemical conversion film with the paint film is degraded, and further water deposited on the paint film passe through the vacancies and reaches the steel surface to cause the generation of rust.